In recent years, with the incessant demand for increasingly smaller active circuit elements in semiconductor integrated circuits and other microelectronic devices, there has been an urgent demand for projection-microlithography systems capable of achieving pattern transfer with increasingly greater resolution of fine circuit patterns. In this regard, the resolution limitations of optical (VUV) microlithography have become burdensome, resulting in an extensive current effort to develop a practical “next generation lithography” (NGL) system. Promising NGL approaches are directed to the use of wavelengths substantially shorter than VUV, namely the X-ray wavelengths.
An X-ray projection-microlithography system (i.e., an X-ray “transfer-exposure” apparatus) typically comprises an X-ray source, an illumination-optical system, a reticle stage (for holding a pattern-defining pattern to be transfer-exposed), an image-formation-optical (projection-optical) system, and a substrate stage (for holding a lithographic substrate such as a semiconductor wafer coated with a suitable “resist”). An especially large amount of expense and effort has been directed to the development of a practical SXR (EUV) microlithography system that utilizes an exposure wavelength in the range of 10-15 nm. These systems are known in the art as “EUVL” (EUV lithography) systems.
In the development of EUVL systems, particular attention has been directed to the development of a practical EUV light source. Originally synchrotron radiation (SR) sources were used. However, SR sources are extremely large and expensive and provide EUV light intensity that is too low for practical use in lithography. Hence, substantial effort currently is being directed to the development of laser-plasma EUV sources and discharge-plasma EUV sources. The currently required intensity from an EUV source is 50-150 W at a wavelength in the range of 13-14 nm and a bandwidth of 2% (approximately 0.3 nm). These sources are pulsatile, and the current exposure-control requirements demand a pulse frequency of 5 KHz or more for best exposure uniformity, etc.
Another important variable concerning an EUV source is “etendue,” which is a product of the area of the light source and the solid angle of radiation from the source. Desirably, the etendue does not exceed the product of the area of the illumination region and the solid angle of the illumination beam. In an EUVL system the area of the illumination region as well as the solid angle of the illumination beam are smaller than the area and solid angle, respectively, of the illumination beam in a conventional optical microlithography system. Desirably, the etendue is approximately 1 mm2 str or less. If etendue were allowed to increase above this limit, it theoretically would become impossible to guide such a beam to an illumination region.
In a discharge-plasma EUV source an electrode material or other substance (“target substance”) is situated or placed in the vicinity of electrodes between which an electrical discharge is created. The discharge creates a plasma that, in the presence of the target substance, produces SXR radiation. An advantage of discharge-plasma EUV sources is simplicity and compactness of construction, compared to laser-plasma EUV sources and SR EUV sources. Also, the amount of SXR radiation emitted from a discharge-plasma EUV source is relatively large, due in part to the relatively high efficiency with which SXR radiation is produced per unit of electrical power consumed by the source. These sources also are low in cost. Discharge-plasma EUV sources can have any of various specific configurations currently under development, such as a “Z-pinch” configuration, a “capillary” configuration, and a “plasma-focus” configuration.
The maximum output from a discharge-plasma EUV source is limited mainly by the thermal-load limit of the source. In any of the configurations of discharge-plasma EUV sources listed above, a large electrical current passes between the electrodes and from the electrodes through the generated plasma. Since the electrodes and plasma have some electrical resistance, the large current produces substantial electrode heating. In addition, since the plasma is generated in the vicinity of the electrodes, heat from the plasma also is transmitted by irradiation to the electrodes, which further increases the heat load on the electrodes.
In order to reduce SXR absorption by gas in the vicinity of the plasma, discharge-plasma sources that generate SXR (EUV) wavelengths typically are enclosed in a chamber evacuated to a high vacuum. The vacuum environment in the chamber inhibits rapid dissipation of heat from the source by convection or thermal conduction. This inability to dissipate heat from the electrodes causes further heat accumulation in the electrodes.
Increases in electrode temperature are especially pronounced at high pulse frequencies of plasma generation. Under such conditions the temperature increase can be sufficiently high to melt the electrodes, rendering plasma (and thus EUV) generation impossible.
By limiting this type of thermal load on a discharge-plasma EUV source, the achievable EUV output from the source is limited to approximately 20-30 W, which is substantially less than the EUV output (50-150 W) required by an EUVL system.
A conventional manner of increasing the EUV output from the source involves “bundling” multiple individual discharge-plasma EUV sources 7, as shown in FIG. 1. However, as shown in FIG. 2, merely bundling multiple sources 7 in this manner increases the effective width of the collective light source to an extent that the etendue of EUV light produced by the bundled source is too large for effective utilization of the EUV beam from the source. FIG. 2(a) depicts an end view (axial view) of a single discharge-plasma EUV source 7, showing the light-emission portion 8. The diameter of the light-emission portion 8 (approximately 0.1 mm or less) is small compared to the outside diameter of the source 7 (approximately 10 mm or more). The etendue in this instance is determined by the solid angle of the downstream focusing optical system and the area of the light-emission portion 8.
In a bundled source, in contrast, the effective light-source area that determines etendue is not the sum of the areas of the individual light-emission portions, but rather the area of the smallest circle encompassing all of the individual light-emission portions of the respective individual sources. For example, in FIG. 2(b) four individual discharge-plasma EUV sources 7a-7d are bundled. The effective light-source area is the area of the circle 11 circumscribing all the individual light-emission portions 8a-8d. Hence, the etendue of the bundled source is substantially larger than of an individual constituent source of the bundle. As a result, with the conventional bundled source, the output of EUV light that actually can be used is insignificantly increased over that of an individual source.